Electroluminescent (“EL”) cells or segments generally comprise a layer of phosphor material, such as a zinc sulphide powder, housed between two electrodes. Typically although not in every case, one of the cell electrodes can be composed of a transparent material, such as indium tin oxide, deposited onto a transparent substrate, which provides the front surface for the electroluminescent segment. The phosphor layer housed between the two conductors behaves generally like a capacitor, in which the phosphor layer acts as a dielectric insulator. Depending on its size, the effective capacitance of the electroluminescent segment can range anywhere from picofarads to microfarads. Light is radiated from the electroluminescent cell by subjecting the phosphor layer to a strong electric field, causing photoemission as electron-hole pairs separated in the phosphor layer recombine.
To produce light from the electroluminescent cell, a large voltage is applied across its electrodes. In the resulting current flow, charge is accumulated on the cell electrodes due to the capacitive nature of the electroluminescent segment. To prevent the long-term buildup of charge on the cell electrodes, the applied voltage is typically alternated between positive and negative polarities. Any charge that is accumulated across the electroluminescent segment electrodes during a charging phase is subsequently dissipated during a discharging phase and, through alternation of the two discrete phases, effectively continuous photoemission can be maintained within the phosphor layer. Depending on the size of the electroluminescent segment and the desired illumination intensity, an alternating voltage in the range of 100-150 V, and having a frequency in the range of 100-400 Hz, may be appropriate.
Because of their compact size and relatively low current consumption, electroluminescent cells are widely used in small battery operated electronic devices. For example, liquid crystal displays can be backlight using electroluminescent cells in portable communication devices, such as mobile phones, personal data assistants, portable music players, and the like. The output of the batteries used in these electronic devices, however, is commonly a low-level DC voltage in the range of about 1-5 V. Accordingly, some form of voltage converter is often used to convert the low-voltage DC output of the battery into the high-voltage AC current used to driving the electroluminescent cell.